The present invention relates to methods and apparatus for controlling the recirculation of exhaust gas into the air inlet of a pressure boosted internal combustion engine. The invention relates particularly to control devices for exhaust gas recirculation (EGR) in pressure boosted compression ignition or diesel engines.
EGR has been found particularly desirable in eliminating "knock" in a diesel engine at cold idle and at light load conditions, to minimize the formation of oxides of nitrogen (NOx) in the exhaust in order to meet the governmental requirements for exhaust emissions in such engines, particularly in road vehicle applications of diesel engines. Known techniques for providing EGR in vehicular diesel engines have included providing a crossover passage between the engine exhaust pipe and the boosted air inlet, and providing a control valve operative in response to changes in pressure in the engine exhaust pipe, or by mechanical connection to the fuel injector control linkage or by a generated vacuum control signal.
However, where it is desired to provide maximum EGR at engine idle and diminish the amount of EGR as speed and load increase, it has been found cumbersome and difficult to provide adequate control of the EGR valve utilizing exhaust pipe pressure as the source of control signal generation. Alternatively, it has been suggested to provide an auxiliary vacuum pump to generate a control signal suitable for controlling EGR based upon a vacuum proportional to engine speed. However, this has proven to be costly and difficult to install in view of the additional hardware and plumbing for the vacuum pump in high volume mass production of road vehicles.
Thus, it has been desired to provide a way or means of controlling EGR in a pressure boosted engine in such a manner as to provide adequate EGR at engine idle and to control the EGR in accordance with the engine load conditions in a simple economical device which may be readily installed on mass produced engines for vehicular use.